This invention relates to a pole-type antenna unit and, in particular, to a pole-type and personal-type miniature antenna unit for a digital radio receiver for receiving an electric wave from an artificial satellite (that may be called a “satellite wave”) or an electric wave on the ground (that may be called a “terrestrial wave”) to listen in a digital radio broadcasting.
In recent years, a digital radio receiver, which receives the satellite wave or the terrestrial wave to listen the digital radio broadcasting, has been developed and is put to practical use in the United States of America. The digital radio receiver is generally mounted on a mobile station, such as an automobile, and can receive an electric wave having a frequency of about 2.3 gigahertz (GHz) to listen in a radio broadcasting. That is, the digital radio receiver is a radio receiver which can listen in a mobile broadcasting. Inasmuch as the received wave has the frequency of about 2.3 GHz, a reception wavelength (resonance frequency) λ thereof is equal to about 128.3 mm. In addition, the terrestrial wave is an electric wave in which a signal where the satellite wave is received in an earth station is frequency shifted a little and is retransmitted in a linearly polarized wave. That is, the satellite wave is a circularly polarized wave, while the terrestrial wave is the linearly polarized wave.
As described above, since the electric wave having the frequency of about 2.3 GHz is used in the digital radio broadcasting, an antenna unit for receiving such an electric wave should be installed outdoors.
As digital radio receivers, there are a type adapted to be mounted in an automobile, a type adapted to be installed in a house or the like, and a type that is portable using a battery as a power source.
As a specific example of the portable digital radio receiver, there is available a portable electronic device such as a portable sound device. This portable electronic device comprises, in addition to a digital tuner for listening to the digital radio broadcasting, for example, an optical disk drive for reproducing an optical disk such as a compact disk (CD), an amplifier, and a speaker, which are integrally incorporated in a case.
On the other hand, there have been proposed antennas with various structures that are adapted to receive the electric wave having the frequency of about 2.3 GHz. Based on the shapes, they are roughly classified into a planar type (plate type) such as a patch antenna and a cylindrical type such as a loop antenna or a helical antenna. Such an antenna of the planar or cylindrical type is prepared as a separate member from the case of the foregoing portable electronic device and is connected to the digital radio tuner incorporated in the case through a cable and a connector so as to be used.
Generally, the antennas of the cylindrical type are more used than the antennas of the planar type because a wider directivity can be achieved by making the shape of the antenna cylindrical. As described above, the antennas of the cylindrical type are roughly classified into the loop antenna and the helical antenna.
As the loop antenna, there is known an electromagnetic coupling type four-point feeding loop antenna (see, e.g. Patent Document 1: Japanese Unexamined Patent Application Publication (JP-A) No. 2003-298335). The electromagnetic coupling type four-point feeding loop antenna disclosed in Patent Document 1 comprises a hollow cylindrical member formed by rolling a flexible insulating film member into a hollow cylinder about a center axis, a loop portion formed into a loop about the center axis on the hollow cylindrical member along its peripheral surface, and four feed lines for power feeding to the loop portion formed on the peripheral surface of the hollow cylindrical member. Four electromagnetic coupling lines are connected to the loop antenna such that each of them extends along the corresponding one of the four feed lines from the loop portion with a gap defined therebetween, thereby carrying out the power feeding through electromagnetic coupling. This loop antenna has a ground conductor pattern formed on the back of a circuit board extending in a direction perpendicular to the center axis.
On the other hand, the helical antenna is also known (see, e.g. Patent Document 2: Japanese Unexamined Patent Application Publication (JP-A) No. 2003-37430). Patent Document 2 proposes to produce a flexible insulating film member having one surface printed with an antenna pattern composed of four helical conductors (hereinafter referred to as an “antenna pattern printed insulating film member”) and then roll the antenna pattern printed insulating film member into a hollow cylinder about a center axis such that the foregoing one surface becomes an outer peripheral surface, thereby manufacturing a helical antenna. This helical antenna also has a ground conductor pattern formed on the back of a circuit board extending in a direction perpendicular to the center axis.
In the case of each of the foregoing antennas of the cylindrical type, after a satellite wave (circularly polarized wave) is received from the loop portion through the four electromagnetic coupling lines or by the helical conductors as a plurality of received waves, the received waves are phase-shifted by a phase shifter so as to be matched (adjusted) in phase, thereby obtaining a combined wave, and then the combined wave is amplified by a low-noise amplifier (LNA) and sent to a receiver body. Herein, a combination of the helical antenna, the phase shifter, and the LNA is called an antenna unit.
On the other hand, there has also been proposed an antenna unit comprising a helical antenna in the form of an antenna pattern formed on an outer peripheral surface of a cylindrical member, and a phase shifter in the form of a phase shifter pattern formed on the outer peripheral surface of the cylindrical member so as to be continuous with (connected to) the antenna pattern (see, e.g. Japanese Unexamined Patent Application Publication (JP-A) No. 2001-339228).
Such an antenna unit is placed in a topped hollow cylindrical cover case (cylinder) in order to prevent water invasion. Accordingly, the external appearance of the overall antenna unit exhibits a pole shape. In view of this, the antenna unit having such external appearance is called a pole-type antenna unit. Since the pole-type antenna unit is used while being carried, i.e. clipped to a pocket or the like, it is disposed in close proximity to the human body.
In the case where the foregoing electromagnetic coupling type four-point feeding loop antenna is used as the antenna of the cylindrical type, electric wave radiation of the same intensity occurs in upward and downward directions if there is no ground conductor pattern with a certain size extending in the direction perpendicular to the center axis. Specifically, with respect to cross polarization, assuming that there is left-hand circular polarization in the upward direction, there is, in the downward direction, right-hand circular polarization whose intensity is equal to that of the left-hand circular polarization. However, in the pole-type antenna unit, there is no space for providing such a ground conductor pattern extending in the direction perpendicular to the center axis.
Further, as described above, in the electromagnetic coupling type four-point feeding loop antenna, the power feeding to the loop portion is electromagnetically carried out from the four electromagnetic coupling lines through the four feed lines with the gaps defined therebetween. Therefore, the gaps should be accurately set and, thus, there is a problem that it is complicated to achieve impedance matching.
Moreover, when applied to any of mobile, vehicular, and portable receivers, the pole-type antenna unit is required to have as small a size as possible particularly in the center axis direction of the pole.
In addition, when applied to the portable receiver, the influence of a human body, for example, a shift in resonant frequency, tends to occur while the receiver is held in hand.